1. Field of the Invention
This invention relates to a process for the production of an optical glass article and more particularly, it is concerned with a process of economically making a high quality glass material having a three-dimensional refractive index distribution in the glass body, which can be used as a lens for a camera, precision optical device or optoelectronic instrument. In particular, this process is suitable for obtaining a rod lens having a refractive index which varies in the radius direction, as a lens of refractive index distribution type.
2. Description of the Prior Art
Rod lenses of graded type with a parabolic distribution of refractive index, whose refractive index gradually decreases from the center to the outside in the radius direction, have lately been used increasingly as optical parts in duplicating machines, facsimile devices and optical communications. These rod lenses are made of glass materials or plastic materials and some of them have been put to practical use ("Nikkei Electronics" 1979, 8/20, page 64-74 or "Kogyo Zairyo" 1980, Vol. 20, No. 10, page 85-96). In particular, Selfoc (trademark) lenses made by the ion exchange method using multicomponent glasses are well known.
However, such an ion exchange method is essentially limited by the diffusion speed of a network modifier ion, the role of which is to change the refractive index under such a temperature ccndition that glass itself is not deformed. Thus, a cation with a high diffusion speed should be used and for example, a monovalent cation such as Tl, Cs, Rb, K, Na or Li ion can only be used as the network modifier ion. Therefore, it is not easy to reduce the dispersion of the refractive index. Tl should be used for the purpose of increasing the differences of refractive indexes, but handling of Tl is difficult because of its poisonous character. Furthermore, when it is desired to obtain a rod lens with a large diameter, e.g. larger than 3 mm.phi., the ion exchange method is not suitable for practice on a commercial scale, since a long time is taken for ion exchange at a temperature at which glass is not deformed or broken.
In addition, the molecular stuffing method based on another principle has been proposed as a method whereby the above described disadvantage can be overcome. This method comprises immersing in a stuffing solution containing CsNO.sub.3 or the like a porous glass prepared through steps of phase separation, leaching out and washing to precipitate CsNO.sub.3 as a dopant in the micropores of the glass, and changing stepwise the concentration and temperature of the stuffing solution so that in the glass body, the concentration of Cs.sub.2 O will be in a parabolic distribution from the center to the outside, as disclosed in Japanese Patent Application (OPI) Nos. 28339/1975, 12607/1976 and 102324/1978.
However, the above described molecular stuffing method has the disadvantage that in a porous glass obtained by the steps of phase separation, leaching out and washing, there are disorders of the concentration distribution of Cs.sub.2 O and refractive index distribution due to the disorder of the pore diameter distribution, i.e. the growth of abnormality of connected micropores formed in the step of leaching-out. Since the porosity of such a porous glass is less than 50% and it is difficult to increase it, the difference of refractive indexes in a finally obtained glass body amounts to at most about 2.0% even if an aqueous solution of CsNO.sub.3 with a high concentration is used. Accordingly, it has been eagerly desired to develop a porous glass having connected micropores with a desirable and uniform pore diameter distribution.